全文获取类型
收费全文 | 859篇 |
免费 | 13篇 |
国内免费 | 3篇 |
专业分类
化学 | 627篇 |
晶体学 | 8篇 |
力学 | 7篇 |
数学 | 94篇 |
物理学 | 139篇 |
出版年
2022年 | 2篇 |
2021年 | 12篇 |
2020年 | 17篇 |
2019年 | 13篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 14篇 |
2015年 | 11篇 |
2014年 | 19篇 |
2013年 | 38篇 |
2012年 | 37篇 |
2011年 | 48篇 |
2010年 | 23篇 |
2009年 | 33篇 |
2008年 | 44篇 |
2007年 | 48篇 |
2006年 | 43篇 |
2005年 | 41篇 |
2004年 | 40篇 |
2003年 | 31篇 |
2002年 | 43篇 |
2001年 | 26篇 |
2000年 | 23篇 |
1999年 | 8篇 |
1998年 | 12篇 |
1997年 | 15篇 |
1996年 | 16篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 12篇 |
1984年 | 16篇 |
1983年 | 2篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 12篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 2篇 |
排序方式: 共有875条查询结果,搜索用时 15 毫秒
161.
The problem of estimating large covariance matrices of multivariate real normal and complex normal distributions is considered when the dimension of the variables is larger than the number of samples. The Stein–Haff identities and calculus on eigenstructure for singular Wishart matrices are developed for real and complex cases, respectively. By using these techniques, the unbiased risk estimates for certain classes of estimators for the population covariance matrices under invariant quadratic loss functions are obtained for real and complex cases, respectively. Based on the unbiased risk estimates, shrinkage estimators which are counterparts of the estimators due to Haff [L.R. Haff, Empirical Bayes estimation of the multivariate normal covariance matrix, Ann. Statist. 8 (1980) 586–697] are shown to improve upon the best scalar multiple of the empirical covariance matrix under the invariant quadratic loss functions for both real and complex multivariate normal distributions in the situation where the dimension of the variables is larger than the number of samples. 相似文献
162.
We report the generation of a continuous-wave squeezed vacuum resonant on the Rb D1 line (795 nm) using periodically poled KTiOPO4 (PPKTP) crystals. With a frequency doubler and an optical parametric oscillator based on PPKTP crystals, we observed a squeezing level of -2.75+/-0.14 dB and an antisqueezing level of +7.00+/-0.13 dB. This system could be utilized for demonstrating storage and retrieval of the squeezed vacuum, which is important for the ultraprecise measurement of atomic spins as well as quantum information processing. 相似文献
163.
Alonso E Fuwa H Vale C Suga Y Goto T Konno Y Sasaki M LaFerla FM Vieytes MR Giménez-Llort L Botana LM 《Journal of the American Chemical Society》2012,134(17):7467-7479
Gambierol is a potent neurotoxin that belongs to the family of marine polycyclic ether natural products and primarily targets voltage-gated potassium channels (K(v) channels) in excitable membranes. Previous work in the chemistry of marine polycyclic ethers has suggested the critical importance of the full length of polycyclic ether skeleton for potent biological activity. Although we have previously investigated structure-activity relationships (SARs) of the peripheral functionalities of gambierol, it remained unclear whether the whole polycyclic ether skeleton is needed for its cellular activity. In this work, we designed and synthesized two truncated skeletal analogues of gambierol comprising the EFGH- and BCDEFGH-rings of the parent compound, both of which surprisingly showed similar potency to gambierol on voltage-gated potassium channels (K(v)) inhibition. Moreover, we examined the effect of these compounds in an in vitro model of Alzheimer's disease (AD) obtained from triple transgenic (3xTg-AD) mice, which expresses amyloid beta (Aβ) accumulation and tau hyperphosphorylation. In vitro preincubation of the cells with the compounds resulted in significant inhibition of K(+) currents, a reduction in the extra- and intracellular levels of Aβ, and a decrease in the levels of hyperphosphorylated tau. In addition, pretreatment with these compounds reduced the steady-state level of the N-methyl-D-aspartate (NMDA) receptor subunit 2A without affecting the 2B subunit. The involvement of glutamate receptors was further suggested by the blockage of the effect of gambierol on tau hyperphosphorylation by glutamate receptor antagonists. The present study constitutes the first discovery of skeletally simplified, designed polycyclic ethers with potent cellular activity and demonstrates the utility of gambierol and its synthetic analogues as chemical probes for understanding the function of K(v) channels as well as the molecular mechanism of Aβ metabolism modulated by NMDA receptors. 相似文献
164.
Yoshihiko Mizushima 《Optical Review》2012,19(5):290-293
Instability of traveling IR waves within a waveguide structure is discussed. A practical model of a traveling wave amplifier of a solid state is proposed, utilizing an optical waveguide. The mechanism of instability is interpreted in terms of an interaction between a plasmon wave and a circuit one under a constraining boundary condition. Properties of the traveling amplification and related problems are discussed, with appropriately suggested semiconductor materials and device designs. The features of the amplifier are a simple structure, a low DC biasing power dissipation for room-temperature operation, unidirectionality, and a wide wavelength range from IR or submillimeter order, suited to various applications. 相似文献
165.
Y Yamamoto K Fukatsu H Nishiyama 《Chemical communications (Cambridge, England)》2012,48(64):7985-7987
The first catalytic synthesis of exocyclic 1,3-dienylphosphine oxides was achieved by the ruthenium-catalyzed selective hydrophosphinylative cyclization of 1,6-diynes. A plausible mechanism involving a ruthenacyclopentatriene intermediate is proposed on the basis of the DFT calculations of model ruthenium complexes. 相似文献
166.
Takeya H Miyazawa K Kato R Wakahara T Ozaki T Okazaki H Yamaguchi T Takano Y 《Molecules (Basel, Switzerland)》2012,17(5):4851-4859
We synthesized superconducting fullerene nanowhiskers (C(60)NWs) by potassium (K) intercalation. They showed large superconducting volume fractions, as high as 80%. The superconducting transition temperature at 17 K was independent of the K content (x) in the range between 1.6 and 6.0 in K-doped C(60) nanowhiskers (K(x)C(60)NWs), while the superconducting volume fractions changed with x. The highest shielding fraction of a full shielding volume was observed in the material of K(3.3)C(60)NW by heating at 200 °C. On the other hand, that of a K-doped fullerene (K-C(60)) crystal was less than 1%. We report the superconducting behaviors of our newly synthesized K(x)C(60)NWs in comparison to those of K(x)C(60) crystals, which show superconductivity at 19 K in K(3)C(60). The lattice structures are also discussed, based on the x-ray diffraction (XRD) analyses. 相似文献
167.
Nishikimi A Uruno T Duan X Cao Q Okamura Y Saitoh T Saito N Sakaoka S Du Y Suenaga A Kukimoto-Niino M Miyano K Gotoh K Okabe T Sanematsu F Tanaka Y Sumimoto H Honma T Yokoyama S Nagano T Kohda D Kanai M Fukui Y 《Chemistry & biology》2012,19(4):488-497
Highlights? CPYPP binds to DOCK2 DHR-2 domain and inhibits its catalytic activity ? CPYPP inhibits DOCK2-mediated Rac activation in cells ? The structural features of CPYPP required for its inhibitory effect were revealed ? CPYPP inhibits lymphocyte migration and activation in vitro and in vivo 相似文献
168.
Portfolio optimization problem under concave transaction costs and minimal transaction unit constraints 总被引:9,自引:0,他引:9
We will propose a branch and bound algorithm for calculating a globally optimal solution of a portfolio construction/rebalancing
problem under concave transaction costs and minimal transaction unit constraints. We will employ the absolute deviation of
the rate of return of the portfolio as the measure of risk and solve linear programming subproblems by introducing (piecewise)
linear underestimating function for concave transaction cost functions. It will be shown by a series of numerical experiments
that the algorithm can solve the problem of practical size in an efficient manner.
Received: July 15, 1999 / Accepted: October 1, 2000?Published online December 15, 2000 相似文献
169.
Tolerance bioaccumulation and biotransformation of arsenic compounds by a freshwater prawn (Macrobrachium rosenbergii) were investigated. M. rosenbergii was exposed to 10, 20, 30 and 35 μg As cm−3 of disodium arsenate [abbreviated as As(V)], 25, 50, 100 and 120 μg As cm−3 of methylarsonic acid (MMAA), or 100,200, 300 and 350 μg As cm−3 of dimethylarsinic acid (DMAA). Tolerances (50% lethal concentration: LC50) of the prawn against As(V), MMAA, and DMAA were 30, 100, and 300 μg As cm−3, respectively. The prawn accumulated arsenic compounds directly from aqueous phase and biotransformed them in part. Both methylation and demethylation of the arsenicals were observed in vivo. Highly methylated and less toxic arsenicals were less accumulated in M. rosenbergii. 相似文献
170.
Dr. Yoshinori Tokairin Yuhei Shigeno Dr. Jianlin Han Prof. Dr. Gerd-Volker Röschenthaler Dr. Hiroyuki Konno Dr. Hiroki Moriwaki Vadim A. Soloshonok 《ChemistryOpen》2020,9(1):93-96
Four differently substituted chiral Ni(II)-complexes of dehydroalanine Schiff base were prepared and reacted with BrCF2COOEt/Cu under the standard reaction conditions. The observed diastereoselectivity was found to depend on the degree and pattern of chlorine substitution for hydrogen in the structure of the dehydroalanine complexes. The unsubstituted complex gave the ratio of diastereomers (S)(2S)/(S)(2R) of 66/34. On the other hand, introduction of chlorine atoms in the strategic positions on the chiral ligands allowed to achieve a practically attractive diastereoselectivity of (∼98.5/1.5). Diastereomerically pure major product was disassembled to prepare 9-fluorenylmethyloxycarbonyl (Fmoc) derivative of (S)-4,4-difluoroglutamic acid. 相似文献